Solid state light sources may be used to provide colored (e.g., non-white) or white light (e.g., perceived as being white or near-white). A solid state lighting device may include, for example, at least one organic or inorganic light emitting diode (“LED”) or a laser. White solid state emitters have been investigated as potential replacements for white incandescent or fluorescent lamps due to reasons including substantially increased efficiency and longevity. Longevity of solid state emitters is of particular benefit in environments where access is difficult and/or where change-out costs are extremely high.
A solid state lighting device produces light (ultraviolet, visible, or infrared) by exciting electrons across the band gap between a conduction band and a valence band of a semiconductor active (light-emitting) layer, with the electron transition generating light at a wavelength that depends on the band gap. Thus, the color (wavelength) of the light emitted by a solid state emitter depends on the materials of the active layers thereof. Solid state light sources provide potential for very high efficiency relative to conventional incandescent or fluorescent sources, but present significant challenges in simultaneously achieving good efficacy, good color reproduction, and color stability (e.g., with respect to variations in operating temperature).
Color reproduction is commonly measured using Color Rendering Index (CRI) or average Color Rendering Index (CRI Ra). In the calculation of the CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference illuminant and the test source. The difference in color appearance ΔEi, for each sample, between the test and reference illumination, is computed in CIE 1964 W*U*V* uniform color space. It therefore provides a relative measure of the shift in surface color and brightness of an object when lit by a particular lamp. The general color rendering index CRI Ra is a modified average utilizing the first eight indices, all of which have low to moderate chromatic saturation. The CRI Ra equals 100 (a perfect score) if the color coordinates and relative brightness of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80) for general illumination use where the colors of objects are not important. For some general interior illumination, a CRI Ra>80 is acceptable. CRI Ra>85, and more preferably, CRI Ra>90, provides greater color quality.
CRI only evaluates color rendering, or color fidelity, and ignores other aspects of color quality, such as chromatic discrimination and observer preferences. The Color Quality Scale (CQS) developed by National Institute of Standards and Technology (NIST) is designed to incorporate these other aspects of color appearance and address many of the shortcomings of the CRI, particularly with regard to solid-state lighting. The method for calculating the CQS is based on modifications to the method used for the CRI, and utilizes set of 15 Munsell samples having much higher chroma than the CRI indices.
Aspects relating to the present inventive subject matter may be better understood with reference to the 1931 CIE (Commission International de I'Eclairage) Chromaticity Diagram, which is well-known and readily available to those of ordinary skill in the art. The 1931 CIE Chromaticity Diagram maps out the human color perception in terms of two CIE parameters x and y. The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors.
The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation: E(λ)=A λ−5/(eB/T−1), where E is the emission intensity, λ is the emission wavelength, T the color temperature of the blackbody, and A and B are constants. Color coordinates that lie on or near the blackbody locus yield pleasing white light to a human observer. The 1931 CIE Diagram includes temperature listings along the blackbody locus (embodying a curved line emanating from the right corner). These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants which produce light that is on or near the blackbody locus can thus be described in terms of their color temperature.
The term “white light” or “whiteness” does not clearly cover the full range of colors along the BBL since it is apparent that a candle flame and other incandescent sources appear yellowish, i.e., not completely white. Accordingly, the color of illumination may be better defined in terms of correlated color temperature (CCT) and in terms of its proximity to the BBL. The pleasantness and quality of white illumination decreases rapidly if the chromaticity point of the illumination source deviates from the BBL by a distance of greater than 0.01 in the x, y chromaticity system. This corresponds to the distance of about 4 MacAdam ellipses, a standard employed by the lighting industry. A lighting device emitting light having color coordinates that are within 4 MacAdam step ellipses of the BBL and that has a CRI Ra>80 is generally acceptable as a white light for illumination purposes. A lighting device emitting light having color coordinates within 7 MacAdam ellipses of the BBL and that has a CRI Ra>70 is used as the minimum standards for many other white lighting devices including compact fluorescent and solid state lighting devices.
General illumination generally has a color temperature between 2,000 K and 10,000 K, with the majority of lighting devices for general illumination being between 2,700 K and 6,500 K. The white area proximate to (i.e., within approximately 8 MacAdam ellipses of) of the BBL and between 2,500 K and 10,000 K, is shown in FIG. 10 (based on the 1931 CIE diagram).
Illumination with a CRI Ra of less than 50 is very poor and only used in applications where there is no alternative for economic issues. Lights with a CRI Ra between 70 and 80 have application for general illumination where the colors of objects are not important. For some general interior illumination, a CRI Ra>80 is acceptable. A light with color coordinates within 4 MacAdam step ellipses of the Planckian locus and a CRI Ra>85 is more suitable for general illumination purposes. CRI Ra>90 is preferable and provides greater color quality.
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), and light emitting diodes are inherently narrow-band emitters, no single light emitting diode junction has been developed that can produce white light. A representative example of a white LED lamp includes a blue LED chip (e.g., made of InGaN and/or GaN), arranged to stimulate a phosphor one or more phosphors (e.g., commonly yellow phosphors such as YAG:Ce or BOSE). A portion of the emissions of the blue LED chip pass through the phosphor(s), while another portion of such emissions is absorbed by the phosphor(s), which becomes excited and emits yellow emissions. The resulting mixture of blue and yellow light (sometimes termed ‘blue shifted yellow’ or ‘BSY’ light) may be perceived as cool white light. Cool white LEDs have a color temperature of approximately 5000K, which is generally not visually comfortable for general illumination, but may be desirable for the illumination of commercial goods or advertising and printed materials. Various methods exist to enhance cool white light to increase its warmth, including supplementation with a red LEDs or red phosphor. Additional or different supplemental LEDs and/or phosphors (e.g., of other colors) may be used, including but not limited to LEDs and/or phosphors having peak wavelengths in the long wavelength blue, cyan, green, yellow, amber, orange, and red spectral regions.
Various methods exist to enhance cool white light to increase its warmth. Acceptable color temperatures for indoor use are typically in a range of from 2700-3500K; however, warm white LED devices are typically significantly less efficient than cool white LED devices. To promote warm white colors, an orange phosphor or a combination of a red phosphor (e.g., CaAlSiN3 (‘CASN’) based phosphor) and yellow phosphor (e.g., Ce:YAG or YAG:Ce) can be used in conjunction with a blue LED. A green phosphor (e.g. LuAG:Ce) may additionally or alternatively be provided. Cool white emissions from a BSY element (including a blue emitter and yellow phosphor) may also be supplemented with a red LED, such as disclosed by U.S. Pat. No. 7,095,056 to Vitta, et al. and U.S. Pat. No. 7,213,940 to Negley et al. (with each of the foregoing patents being hereby incorporated by reference herein), to provide warmer light. While such devices permit the correlative color temperature (CCT) to be changed, the CRI of such devices may be reduced significantly at higher color temperatures.
To provide enhanced color rendering, multiple lumiphors (e.g., phosphors) may be arranged to be stimulated by one or more solid state emitters. For example, one or more short wavelength solid state emitters (e.g., blue LED) may be used to stimulate emissions from a mixture of lumiphoric materials or discrete layers of lumiphoric material including red, yellow, and green lumiphoric materials. Use of red, yellow, and green lumiphoric materials arranged to be stimulated by blue LEDs is disclosed, for example, in U.S. Patent Application Publication No. 2011/0220920, which is hereby incorporated by reference as if set forth fully herein.
Use of red and blue LEDs in the same device entails additional issues, since phosphide-based red LEDs exhibit substantially different changes in intensity and/or chromaticity than nitride-based blue LEDs with respect to changes in device operating temperature. That is, red LEDs include active regions typically formed of Group III phosphide (e.g., (Al,In,Ga)P) material, in contrast to blue LEDs, which include active regions typically formed of Group III nitride materials (e.g., represented as (Al,In,Ga)N, including but not limited to InGaN). Group III phosphide materials typically exhibit substantially less temperature stability than Group III nitride materials. Due to their chemistry, red LEDs lose a significant portion (e.g., 40-50%) of their efficacy when operating at 85° C. versus operating at a cold condition (i.e., room temperature or less). When red and blue LEDs are in conductive thermal communication with one another (e.g., affixed to a common substrate or in thermal communication with a common heatsink), heat emanating from the blue LEDs may increase the temperature of the red LEDs. It may be difficult to maintain a relatively constant color point over a wide range of temperatures (including high temperatures) utilizing a device including a Group III-nitride-based blue LED (e.g., as part of a BSY emitter) and Group III-phosphide based red LED without adjusting supply of current to at least one of the BSY element and the red LED, due to the different temperature responses of the blue and red LED.
As an alternative to stimulating a yellow phosphor with a blue LED, another method for generating white emissions involves combined use of red, green, and blue (“RGB”) light emitting diodes in a single package. The combined spectral output of the red, green, and blue emitters may be perceived by a user as white light. Each “pure color” red, green, and blue diode typically has a full-width half-maximum (FWHM) wavelength range of from about 15 nm to about 30 nm. Due to the narrow FWHM values of these LEDs (particularly the green and red LEDs), aggregate emissions from the red, green, and blue LEDs exhibit low color rendering in general illumination applications.
Another example of a known white LED lamp includes one or more ultraviolet (UV)-based LEDs combined with red, green, and blue phosphors. Such lamps typically provide reasonably high color rendering, but exhibit low efficacy due to substantial Stokes losses.
The art continues to seek improved solid state lighting devices that address one or more limitations inherent to conventional devices. For example, it would be desirable to provide solid state lighting devices capable of providing white light in a wider variety of applications, with greater energy efficiency, with improved color rendering over a range of correlative color temperatures, with improved efficacy, with improved color stability at high flux, with reduced size, with enhanced configuration flexibility, and/or with longer duration of service.